Principles of Process Measurements PDF

Principles of Process Measurements Pressure Measurements 1. Primary Elements a) Bourdon Tube – C-type bourdon tubes are usually used in ordinary pressure gauge. Helical and Spiral type are used for high pressure applications. b) Diaphragm – a d...

Principles of Process Measurements Pressure Measurements 1. Primary Elements a) Bourdon Tube – C-type bourdon tubes are usually used in ordinary pressure gauge. Helical and Spiral type are used for high pressure applications. b) Diaphragm – a device usually made of rubber which creates a partition between two areas. It usually moves and reacts due to pressure applied on it. c) Bellows – a device that expands or retract due to pressure.  d) Strain gauge – a device which changes resistance proportional to the pressure applied on it. An increase in pressure results in an increase in resistance. e) Piezoelectric Transducer – a device which is composed of a quartz crystal that when subjected to pressure creates a very small electrical signal proportional to the amount of force exerted to it. = f) Variable Capacitance Transducer – a device consist of two plates wherein the capacitance is varied by the change in the distance of the plates. The distance between the two plates is proportional to amount of pressure that acts on the area of the plates. 2. Units of pressure – There are lots of different pressure units being used on- board a ship. For SI units the basic unit is Pascal which is very small. The  most widely used unit of pressure is BAR. In English system, PSI is most widely used. A conversion table is shown below. 3. Local Reference – A pressure gauge serves as a local indication for pressure measurement. For differential pressure, a U-tube manometer is often used. If ever a pressure alarm is indicated in the control system, we checked if it is true by using these instruments as reference. 4. Remote Control – Pressure switch and pressure transmitters are used to measure pressure in the system and gives a corresponding signal to the controller. Sometimes, switches are used to give alarm signals or as an emergency switch to cut-off electrical signals. Pressure transmitter gives a standard output of 4 to 20 mA or 0.2 to 1.0 Bar.